Copolymer of ethylene oxide and at least one substituted oxirane carrying a cross-linkable function, process for preparation thereof, and use thereof for producing ionically conductive materials

ABSTRACT

The invention concerns a copolymer of ethylene oxide and at least one substituted oxirane carrying a cross-linkable function. The copolymer comprises ethylene oxide, —O—CH 2 —CHR— units in which R is a substituent containing a reactive function which is cross-linkable by free radical process, and possibly —O—CH 2 —CHR′— units in which R′ is a substituent containing no reactive function which is cross-linkable by means of a free radical process, It is characterized by an excellent polydispersity index I=M W /Mn and a random distribution of the different monomer units. The copolymer is prepared by an anionic copolymerization process. The copolymer is useful for preparing a solid electrolyte having good mechanical properties, a good cationic conductivity and a good chemical compatibility with the electrodes of a generator operating with alkali metals such as lithium and sodium.

CROSS-REFERENCE

This application is a continuation-in-part of U.S. application Ser. No.08/350,029 filed Nov. 29, 1994.

BACKGROUND OF INVENTION

a) Field of the invention

The present invention concerns a copolymer of ethylene oxide and atleast one substituted oxirane carrying a cross-linkable function, aprocess for the preparation thereof and the use thereof for producing asolid electrolyte having good mechanical properties, a good cationicconductivity and a good chemical compatibility with the electrodes of agenerator which operates with alkali metals, such as lithium and sodium.

b) Description of Prior Art

Solvating polymers are known to be useful for preparing ionicallyconductive materials. Polymers of ethylene oxide or of dioxolane arepolymers which solvate cations, in particular alkali cations such as forexample the ion Li⁺ which is present in rechargeable electrochemicalgenerators of the polymer electrolyte lithium battery type. However,these polymers are semi-crystalline, since crystallinity varies as afunction of the molecular weight of the polymer. This semi-crystallinecharacter of the polymers results in a decrease of the conductivity ofthe materials in which the polymers are present.

It has been found that it was possible to decrease the crystallinity ofsemi-crystalline polymers, without affecting their solvating propertiesand their electrochemical stability, by introducing defects in themacromolecular chain at possibly irregular intervals. However, it hasbeen observed that the introduction in a semi-crystalline polymer, suchas for example a high molecular weight, polyoxyethylene (POE), of unitsproducing disparities, i.e. substituting a semi-crystalline polymer witha copolymer or a polycondensate, is frequently accompanied by a decreaseof the molecular weights and a lowering of the mechanical properties,for example at high temperature. An attempt was made to overcome thisdisadvantage by introducing into the polymer, units which contribute tothe formation of tri-dimensional networks by cross-linking thecopolymer, before or after its formation. Because of the restraintsimposed by the requirements of electrochemical stability, theparticularly preferred units permitting cross-linking are selected amongthose which contain an unsaturated carbon/carbon bond, such as an allylbond or a vinyl bond. The introduction of such units into a copolymer,additionally makes it possible to fix various groups, for example ionicgroups, on the macromolecular chain.

Using an initiator based on organometallic derivatives of non-alkali andnon-alkali-earth metals, for example an alkyl aluminum or an alkyl zinc,it is possible to prepare copolymers of an ethylene oxide and of anoxirane which carries an unsaturated substituent by coordinationpolymerization. See, for example, E. J. Vandenberg, Journal of PolymerScience, Part A-1, Vol. 7, pages 525-567 (1969).

This type of polymerization is not affected by the presence of a smallamount of impurities. However, the reactivity of the various comonomersdepends on their steric hindrance. Thus, in the production of acopolymer of ethylene oxide and an oxirane carrying a saturatedsubstituent (for example propylene oxide) or an oxirane carrying anunsaturated substituent (for example allyl glycidyl ether), thepolymerization yield of ethylene oxide is near 100%, while the yield ofthe substituted oxirane in a copolymer having a molecular weight higherthan 1000 is only 60%. In addition, ethylene oxide is consumedpreferably at the start of the polymerization. Because of the differenceof reactivity of the monomers, the copolymer formed at the start of thepolymerization contains an excess of ethylene oxide and has a highermolecular weight than the one formed during or towards the end of thepolymerization reaction. The copolymer formed by coordinationpolymerization thus has long poly(oxyethylene) sequences which arecrystalline and wherein the molecular weights of these sequences arehighly heterogeneous.

It is also possible to polymerize saturated oxiranes such as ethyleneoxide or propylene oxide through an anionic process. When such apolymerization is carried out with initiators of the sodium hydroxide orpotassium hydroxide type in an aqueous solution or in protic solventssuch as ethylene glycol, a number of transfer reactions towards thesolvent take place, and the molecular weights obtained are very low.When the anionic polymerization of oxiranes is carried out in thepresence of initiators of the potassium alcoholate or cesium alcoholatetype in an aprotic solvent which solvates cations, or in the presence ofcomplexing agents such as crown-ethers, ethylene oxide undergoes aliving polymerization, i.e. the number average degree of polymerization(DPn) increases with the conversion rate, the distribution of molecularweights is narrow, the polydispersity I=Mw/Mn is near 1 and there ispractically no transfer and termination reactions. An anionicpolymerization which is carried out under these conditions enables oneto obtain high molecular weights when the polymer is ethylene oxide.However, when used with monomers of the substituted oxirane type, it hasonly been possible to obtain low molecular weights (M_(W)<20,000) tothis date. For example, the polymerization of styrene oxide initiatedwith potassium tert-butanolate gives a poly(oxystyrene) having a molarweight of 1000 g, and the growth of the poly(oxypropylene) chains isinterrupted by transfer reactions towards the monomer [D-M Simons and J.J. Verbane, J. Polym. Sc. 1960, 44, 303]. When the monomer is phenylglycidyl ether, chain growth is also rapidly interrupted by transfertowards the monomer [C. C. Price, Y. Atarachi, R. Yamamoto, J. Poly.Sci. PartA1, 1969, 7, 569]. In spite of the advantages associated withnearly quantitative conversion rates in the case of anionicpolymerizations, the prior art shows a living character of thepolymerization for ethylene oxide only.

The anionic polymerization of cyclic ethers such as ethylene oxide andits substituted derivatives (propylene oxide, butylene oxide, allylglycidyl ether, etc.) requires a very low level of impurities such aswater alcohol, etc. Copolymerization is slow relative to thehomopolymerization of ethylene oxide, and requires therefore a muchsmaller amount of impurities in order to avoid the chain transfers andchain terminations which yield a low molecular weight copolymer (Mw lessthan 20,000 for example). To obtain a high molecular weight copolymer(Mw larger than 50,000, for example), one normally requires drasticpurification techniques, such as drying and distillation on a vacuumline. Such extreme purification methods are suitable for the preparationof small amounts (1 to 20 g) of model compounds, but would be tooexpensive for the production of industrial quantities (over 100 kg).

A review of the prior art indicates that an electrolyte may consist of acopolymer of ethylene oxide, methyl glycidyl ether and a small amount ofallyl glycidyl ether, the copolymer of course including an ionizablesalt, all as disclosed in Couput U.S. Pat. No. 5,086,351. Similarcopolymers are also disclosed in U.S. Pat. No. 5,206,756 (Cheshire);U.S. Pat. No. 4,578,326 (Armand); U.S. Pat. No. 5,350,646 (Armand); andFrench Patent 2 570 224. However, these copolymers have all beenprepared by coordination copolymerization and thus have defectivemechanical properties, as mentioned above. For example, a simplecalculation with respect to the copolymer described by Cheshire in U.S.Pat. No. 5,206,756, column 25, will show that its polydispersity is 95.The other copolymers of the prior art, produced by coordinationpolymerization, all possess polydispersisty values which are higher thandesirable to produce electrolytes with excellent mechanical properties.

The present invention aims at providing a copolymer of ethylene oxideand of at least one substituted oxirane carrying a cross-linkablereactive function, by a free radical process, which gives an ionicallyconductive material of improved mechanical properties as compared to thematerials obtained from known copolymers of the poly(oxyalkylene) type,without decreasing the ionic conductivity by providing too manycross-linking points which would cause an increase of the glasstransition temperature Tg, said ionically conductive materialadditionally showing an excellent chemical compatibility with theelectrodes of a generator when the material is used as an electrolyte.

SUMMARY OF INVENTION

It is consequently an object of the present invention to provide acopolymer having a chain comprising ethylene oxide units, —O—CH₂—CHR—units in which R is a substituent having a reactive function which iscross-linkable by a free radical process, wherein R may be differentfrom one unit to the other, and possibly —O—CH₂—CHR′— units in which R′is a substituent having no reactive function which is cross-linkable byfree radical process, wherein R′ may be different from one unit to theother. The copolymer according to the present invention is characterizedin that the copolymer has a polydispersity Mw/Mn which is lower than orequal to 2.2, and a random distribution of monomer units, the copolymerbeing prepared by anionic copolymerization in an aprotic solvent and inthe presence of an anionic polymerization initiator, and wherein themonomers and solvent used have a water and impurity content lower thanor equal to 100 ppm.

Among the copolymers of the present invention, those which have a weightaverage molecular weight Mw higher than or equal to 20,000, morespecially those which have Mw higher or equal to 100,000, areparticularly interesting.

The copolymers of the present invention in general have a polydispersitywhich is between 1.5 and 2.2.

The different units are distributed at random in the chain of acopolymer of the present invention, however, sequences constituted bythe chain of a same monomer unit are more regular than in copolymersobtained by known processes, i.e. by coordination polymerization. It istherefore relatively easy to foresee the length of the sequences, whichdepends only on the relative proportion of the monomers. The randomdistribution of the monomer units is an important characteristic whenthe copolymer obtained is subjected to grafting in order to fix an ionicgroup on the reactive function of the substituents R of the oxiraneunit. A random distribution of the ionic groups is essential in order toprevent the formation of privileged passageways for the ions, when thecopolymer is used as an ionically conductive material.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the —O—CH₂—CHR— units of a copolymer according to the presentinvention, the reactive function which is cross-linkable by free radicalprocess and which is present in the radical R, is advantageously anunsaturated carbon/carbon bond. The radical R may in this case beselected for example from those having the formulaCH₂═CH—(CH₂)_(q)—(O—CH₂)_(p) in which 1≦q≦6 (q=1 to 6) and p=0 or 1, orthose having the formula CH₃—(CH₂)_(y)—CH═CH—(CH₂)_(x)—(OCH₂)_(p), inwhich 0≦x+y≦5 (x+y=0 to 5) and p=0 or 1. In the same macromolecularchain, it is possible that the unsaturated substituents R are not allidentical. In this description the sign ≦ means smaller or equal to.

In the —O—CH₂—CHR′— units of a copolymer of the present invention, thesubstituent R′ having no reactive function which is cross-linkable byfree radical process may be selected from alkyl radicals, preferablyfrom those having 1 to 16 carbon atoms, more preferably those having 1to 8 carbon atoms.

The substituent R′ may additionally be selected alkoxy radicals such asthe radicals —(CH₂)_(n)—O—((CH₂)_(m)—O)_(p)—CH₃, in which 0≦n≦4, 1≦m≦4and 0≦p≦20; preferably n=1, m=2 and 0≦p≦8. The substituent R′ may alsobe selected from the alkyl(perfluoroalkyl sulfonate) ether radicals;some examples are the radicals having the formula—CH₂—O—(CF₂)_(q)—CF(C_(r)F_(2r+1))—SO₃M, in which M represents a cationof an alkali metal, 0≦q≦4, preferably q=0 or 1, and 0≦r≦4, preferably0≦r≦3; among the preferred radicals of this category, radicals—CH₂—O—CF₂—CF(CF₃)—SO₃M and —CH₂—O—CF₂—SO₃M may be mentioned. Thesubstituent R′ may also be selected among radicals including anionophoric function in which the negative charge is carried by thebis(trifluoromethylsulfonyl) methylide —C(SO₂CF₃)₂M″ carbanion; examplesof these radicals are —CH₂—C(SO₂—CF₃)₂M″ and—(CH₂)_(s)—SO₂—C(SO₂—CF₃)₂M″, in which 1≦s≦16, preferably 0≦s≦8, M″represents a metallic cation, more particularly a monovalent cation suchas a cation of an alkali metal. The radicals —CH₂—C(SO₂—CF₃)₂M″ and—(CH₂)_(s)—SO₂—C(SO₂—CF₃)₂M″ are particularly preferred.

In a same macromolecular chain, the substituents R′ may be not allidentical.

The copolymers of the present invention which have a high weight averagemolecular Mw, i.e., at least equal to 20,000, preferably at least equalto 100,000, are doubly interesting. On the one hand, these copolymershave, in non cross-linked state, intrinsic mechanical propertiessuperior to the copolymers of the same type, of the prior art: they maybe prepared and handled in the form of thin films before cross-linking;they may additionally be used as elastomeric bond or as an adhesive whenassembling components of a generator. If cross-linking is necessary,less cross-linkable functions may be used. On the other hand, a highmolecular weight enables one to use lower quantities of polymerizationinitiator, and limits the number of terminal reactive alcoholate orhydroxyl functions. In the copolymers of the present invention having amolecular weight of at least 20,000, there is consequently a doublelimitation of the concentration and mobility of the reactive chemicalgroups (terminal functions, polymerization initiators, cross-linkablefunctions which are reactive towards alkali metals, low molecular weightpolymers capable of diffusing into the electrodes) which presents amajor interest when a copolymer according to the present invention isused as an ionically conductive material, for example in anelectrochemical generator operating with alkali metals. In this case,the ionic conductivity of an ionically conductive material comprisingthe copolymer is not substantially reduced by cross-linking, because thelow rate of cross-linking has a negligible effect on the glasstransition temperature Tg. In addition, the electrochemicalcompatibility of the ionically conductive material, when used aselectrolyte, with the electrodes of a generator is clearly superior.

In a particular embodiment, a copolymer according to the presentinvention contains at least 70 mole % of oxyethylene units, about 2 toabout 30 mole % of saturated units —O—CH₂—CHR′—, and about 0.05 to about10 mole % of —O—CH₂—CHR— units containing functions which arecross-linkable by free radical process.

The copolymers of the present invention are obtained by anioniccopolymerization, and it is also an object of the invention to provide aprocess for the preparation of said copolymers.

The process for the preparation of a copolymer according to the presentinvention consists in reacting ethylene oxide and one or moresubstituted oxiranes, of which one at least carries a substituent Rcontaining a function which is cross-linkable by a free radical process,in an aprotic solvent in the presence of an anionic polymerizationinitiator, the monomers and the solvent used having a water and impuritycontent lower than or equal to 100 ppm, the reactor used for thepolymerization reaction being free of traces of humidity and impurities.

In the description which follows, an oxirane carrying a substituentcontaining a function which is cross-linkable by free radical processwill be designated by the term “unsaturated oxirane”; an oxirane havinga substituent containing no cross-linkable function will be designatedby the term “saturated oxirane”.

The polymerization initiator is preferably selected from alkali metals,used in metallic form, in the form of an alcoholate or as a complex forexample with a crown-ether. The alkali metal is preferably selected fromcesium and potassium. Potassium alcoholates are particularly preferred.

When the initiator is an alkali metal or an alcoholate of an alkalimetal, the aprotic solvent in which the polymerization is carried out isselected from the group comprising polar solvents. By way of examples ofpolar solvents, one may mention THF, dimethoxyethane anddimethylsulfoxide. However, in view of the fact that ethylene oxide andthe monomers of the oxirane type are polar, a non-polar solvent may alsobe used, for example toluene.

When the initiator is used with a complexing agent such as for example acrown-ether, the aprotic solvent may be a polar solvent or a non polarsolvent such as toluene.

The process of the present invention is carried out with at least oneunsaturated oxirane. Among appropriate unsaturated oxiranes, oxiraneshaving the formula CH₂—CHR—O in which R is as defined above, may bementioned. Allyl glycidyl ether and epoxyhexene are particularlypreferred unsaturated oxiranes.

The function of the units derived from unsaturated oxiranes is topermit, either a cross-linking of the copolymer after the latter hasbeen obtained, or grafting reactions on the substituent, for example inorder to fix ionic groups on the macromolecular chain.

The small amount of impurities in the monomers and in the solvent usedfor the polymerization may be obtained by treating the monomers and thesolvent with molecular sieves, or by distillation or filtration in thecase of ethylene oxide.

The preliminary treatment of the reactor in order to remove impuritiestherefrom may be carried out for example by washing the reactor with asolution containing an initiator, and by removing the initiator solutionbefore the introduction of the reagents.

When carrying out the process of the invention, the reaction mixturecontains very little impurities which would cause chain terminations.The reaction yield is therefore very high, and may reach values near100%. The copolymer obtained thus contains a quantity of residualmonomer which is sufficiently low to make its removal unnecessary. Thisis an important advantage when monomers having a high boiling point (forexample higher than 150° C.) are used.

The process according to the invention may also be used forcopolymerizing ethylene oxide with at least one unsaturated oxirane andat least one saturated oxirane. The introduction of a saturated oxiranein the copolymer enables one to reduce, and even to remove thecrystallinity of the copolymer, and to modify its mechanical properties.Among the saturated oxiranes, those corresponding to the formulaCH₂—CHR′—O, in which R′ is as defined above, may be mentioned.

When a copolymer of the present invention is intended to be used forpreparing an ionically conductive material, it may be useful todeactivate the terminal reactive functions of the macromolecular chains,although the latter may not be numerous because of the high molecularweights.

The terminal functions are in general alcoholate (OH) functions whichare very reactive towards a lithium electrode and which contribute tothe degradation of the polymer electrolyte/lithium electrode interface.The process of the invention may thus advantageously include anadditional step in which the terminal functions are deactivated.

This deactivation may be carried out by means of 2-bromo-1-cyano-ethane,according to the following reaction scheme:PO⁻, K⁺+BrCH₂—CH₂—CN→PO—CH₂—CH₂—CN+KBr

-   -   where PO⁻, K⁺ represent the non deactivated copolymer.

Deactivation of the terminal functions may also be carried out by meansof methyl iodide or methyl sulfate. In this case, the copolymers havemethoxy terminal groups and there is respectively formed potassiumiodide or sulfate, if the terminal function to be disactivated is apotassium alcoholate.

The properties of the copolymers of the present invention make themparticularly useful for preparing materials having ionic conduction. Thehigh molecular weight has a favorable effect on the mechanicalproperties on the one hand and on the electrochemical properties on theother hand, as previously indicated. In addition, the randomdistribution of the unsaturated functions enables one to obtain ahomogeneous cross-linking when these functions are used forcross-linking. If these functions are used for grafting ionic groups onthe copolymer, the statistical distribution of the grafted ionic groupsprevents the formation of privileged passageways for the ions.

In order to prepare an ionically conductive material, copolymers whichcontain at least 70 mole % of ethylene oxide units, about 2 to about 30mole % of units derived from at least 1 saturated oxirane and about 0.05to about 10 mole % of units derived from at least one unsaturatedoxirane may be used. When the material is used without solvent or withlittle solvent (less than 10 weight %), the content of units derivedfrom an unsaturated oxirane is preferably between 0.05 and 1 mole %.When the material is swollen by a solvent, the content of units derivedfrom an unsaturated oxirane may reach up to 10 mole %. When the unitsderived from an unsaturated oxirane are intended to be used for graftingan ionic group on the copolymer, their content is preferably between 3and 5 mole %.

According to one embodiment, an ionically conductive material accordingto the present invention essentially comprises one ionic compound whichis easily dissociable while in solution in a copolymer according to thepresent invention. The ionic compound which is introduced into thecopolymer before cross-linking or into the cross-linked polymer isselected from ionic compounds which are normally used for ionicallyconductive solid polymer materials. By way of example, one may mentionionic compounds A^(a+)Y⁻ _(a) in which A^(a+) represents a proton, ametallic cation, an organic cation of the ammonium, amidinium orguanidinium type, a being the valency of the cation A^(a+); Y⁻represents an anion with delocalized electronic charge, for example Br⁻,ClO₄ ⁻, AsF₆ ⁻, R_(f)SO₃ ⁻, (R_(f)SO₂)₂N⁻, (R_(f)SO₂)₃C⁻, C₆H(_(6-x))(CO(CF₃SO₂)₂C⁻)_(x) or C₆H_((6-x)) (SO₂(CF₃SO₂)₂C—⁻)_(x), wherein R_(f)represents a perfluoroalkyl or perfluoroaryl group, in which 1≦x≦4. Thepreferred ionic compounds are lithium salts, more particularly(CF₃SO₂)₂N⁻Li⁺, CF₃SO₃ ⁻Li⁺, the compoundsC₆H_((6x))—[CO(CF₃SO₂)₂C⁻Li⁺]_(x), in which x is between 1 and 4,preferably x=1 or 2, the compounds C₆H_((6x))—]SO₂(CF₃SO₂)₂C⁻Li⁺]_(x),in which x is between 1 and 4, preferably x=1 or 2. Mixtures of thesesalts with one another or with other salts may be used. By way ofexamples of mixtures of salts, one may mention: (CF₃SO₂)₂N⁻Li⁺ andCF₃SO₃ ⁻Li⁺ or (CF₃SO₂)₂N⁻Li⁺ and C₆H₄—[CO(CF₃SO₂)₂C⁻Li⁺]₂ in variousproportions, but preferably comprising 20 to 40 weight % of(CF₃SO₂)₂N⁻Li⁺. The ionic compound may be incorporated into thecopolymer by immersing the copolymer, possibly in the form of a film,into a solution of the selected ionic compound in a solvent, the solventbeing thereafter evaporated. According to a variant, the ionic compoundmay be incorporated into the copolymer by preparing a film from asolution comprising both the copolymer and the ionic compound.

According to another embodiment, an ionically conductive materialaccording to the present invention essentially consists of a copolymeraccording to the present invention in which an ionic compound containingan unsaturation has been grafted on the radicals R by co-cross-linkingwith the units —CH₂—CHR—O—. In this case, a copolymer according to thepresent invention comprising about 3 to about 5 molar % —CH₂—CHR—O—units is preferably used. Among suitable ionic compounds which may begrafted onto the radicals R, one may use derivatives of perhalogenatedsultones carrying an ionic group described in WO93/16988, for examplecompounds of the type CH₂═CH—CH₂—(CF₂)₂—SO₃M′,CH₂═CH—CH₂—O—CF(C_(y)F_(2y+1))—CF₂SO₃M′ andCH₂═CH—CF(C_(y)F_(2y+1))—CF₂SO₃M′, in which 0≦y≦4, preferably 1≦y≦3, M′represents a proton, a metallic cation, more particularly a cation of amonovalent metal, or an organic cation. Among the metallic cations,those of an alkali metal are particularly preferred. Among the organiccations, one may mention ammonium cations, guanidinium cations andamidinium cations, said organic cations being possibly quarternized. Onemay also mention salts of bis(trifluoromethylsulfonyl)-methylide such as[CH₂═C(CH₃)—CO—C(SO₂—CF₃)₂]⁻Li⁺, [CH₂═C(CH₃)—C(SO₂—CF₃)₂]⁻Li⁺,[CH₂═CH—CH₂—CO—C(SO₂—CF₃)₂]⁻Li⁺, [CH₂═CH—C₆H₄—SO₂—C(SO₂—CF₃)₂]⁻Li⁺,[CH₂═CH—CH₂—SO₂—C(SO₂—CF₃)₂]⁻Li⁺, [CH₂═CH—SO₂—C(SO₂—CF₃)₂ ⁻Li⁺,[CH₂═CH—C₆H₄—CO—C(SO₂—CF₃)₂]⁻Li⁺.

According to yet another embodiment, an ionically conductive materialmay essentially consist of a copolymer according to the presentinvention containing —CH₂—CHR′—O— units in which the radical R′ includesionic groups. The ionic groups may be selected from the group comprisingbis(trifluoro-methylsulfonyl)methylide —C(SO₂CF₃)₂M″ in which M″ is ametallic cation, preferably an alkali, or perfluorosulfonyl groups ofthe type —CH₂—O—(CF₂)_(q)—CF(C_(r)F_(2r+1))—SO₃M, where M represents amonovalent metal. The units —CH₂—CHR′—O— then fulfill two functions. Onthe one hand, they decrease the regularity of the solvatingmacromolecular chain, and consequently its crystallinity; on the otherhand, they confer to the copolymer a cationic unipolar ionic conductorcharacter.

The different means described above for introducing ionic species into acopolymer according to the invention for the preparation of a ionicallyconductive material may of course be combined if desired.

Various additives may be added to the material of the present invention,in order to modify the properties of the final material. Thus, one mayincorporate a plasticizing agent such as ethylene carbonate, propylenecarbonate, y-butyrolactone, dimethylformamide, N-methyl-pyrrolidone,tetraalkylsulfamides, methyl ethers of polyethylene glycols havingmolecular weights between 200 and 2000 and, generally, derivatives ofpolar molecules having a low volatility. The proportion of theseadditives may be up to 1 to 90% of the total weight.

The ionically conductive materials of the invention, consisting of acopolymer and an ionic compound, or of a copolymer carrying ionicsubstituents, can be used as solid polymer electrolyte to separateelectrodes and/or as a component of a composite electrode, especiallywhen the copolymer has a molecular weight at least equal to 20,000. Itis consequently an object of the invention to provide an electrochemicalcell in which the electrolyte comprises an ionically conductive materialaccording to the present invention, and/or in which one at least of theelectrodes is a composite electrode comprising such a material. In aparticular embodiment, the electrolyte is a membrane which separates theelectrodes, the membrane consisting of an ionically conductive materialaccording to the present invention, which is plasticized by the additionof a suitable solvent, for example a mixture of ethylenecarbonate/propylene carbonate (weight ratio about 1/1).

The copolymers and materials with ionic conduction of the presentinvention are useful for an electrochemical generator with alkali metalwhether it is rechargeable or not. Such a generator comprises a negativeelectrode and a positive electrode which are separated by a solidpolymer electrolyte, the solid polymer electrolyte comprising acopolymer according to the present invention. In such a generator, theelectrodes may also contain an ionically conductive material of thepresent invention acting as a conductive binder, when they are preparedin composite form. In this particular application, the copolymers of thepresent invention are particularly interesting because they contain onlya small number of species which can interfere with the electrochemicalreactions. Indeed, the average high molecular weighs obtainedsubstantially decrease the number of reactive ends, and the highpolymerization yield limits the content of residual catalyst. Inaddition, the average high molecular weights that the copolymers maypossess give to the copolymers and the ionically conductive materials inwhich the copolymers are present, an intrinsic mechanical behavior whichis sufficient in the absence of cross-linking. However, if cross-linkingis necessary, the statistical distribution of the cross-linkable unitsenables one to obtain a very homogeneous cross-linking.

The copolymers and the ionically conductive materials are also useful inother electrochemical systems such as in electrochrome systems, systemsfor modulating light, for the preparation of selective membranes orreference membranes in membrane pickups.

The present invention is illustrated by the following examples, it beingunderstood that the invention is not limited to the examples given.

In the following examples, the polymerization was carried out instainless steel Parr® reactors having a 2L capacity, provided with astirrer and a bottom valve enabling one to empty the reactor through thebottom. All the transfer operations were carried out under inertatmosphere by using very dry argon or nitrogen, and without oxygen.

In each example, the reactor was dried by washing it with an initiatorsolution and by removing this initiator solution before introducing thereagents into the reactor.

Ethylene oxide was distilled, the solvents and the other monomers usedwere dried -on molecular sieves before introducing them into thereactor, in order to lower their water content to less than 100 ppm,which value is verified by the Karl-Fischer method.

BRIEF DESCRIPTION OF DRAWINGS

The invention is illustrated but not restricted by means of thefollowing drawings in which:

FIG. 1 is a curve of the variation of the melting temperature T_(m) as afunction of the average molecular weight for PEG, Anio₂₃, Anio₃₄ andAnio₄₅; and

FIG. 2 represents the variation of the conductivity σ as a function ofthe temperature for different electrolytes.

EXAMPLES Example 1

In a reactor which is free from traces of water and impurities, andwhich contains 250 ml of toluene, 10⁻³ mole of potassium tert-butanolateand 3 ml of THF, a mixture of 83.9 g of ethylene oxide, 10 g of methylglycidyl ether and 5.1 g of allyl glycidyl ether was introduced. Thetemperature of the reactor was raised to 120° C. and maintained at thatlevel for 22.5 hours. A decrease of the pressure from 10.3×10⁵ Pa to 10⁵Pa was observed. The temperature was thereafter lowered to 70° C. and100 mg of a sulfide of 3-tert-butyl-4-hydroxy-5-methyl-phenylcommercialized by Aldrich, were added, this product being used as astabilizing and anti-oxidizing agent for the polymer. The reactor wasthereafter washed with a small quantity of toluene and 93 g of copolymerwere recovered after evaporation of the solvent, which corresponds to ayield of 100%. A DSC analysis shows a melting peak T_(m) at 30° C. and aglass transition temperature Tg=−64° C. The weight average molecularweight, determined by steric exclusion chromatography (SEC), is about105,000, with a polydispetsity index of 1.7.

Example 2

In a reactor free from traces of water and impurities and containing 115ml of toluene, 2.10⁻³ mole of potassium tert-butanolate and 6 ml of THF,a mixture of 87.2 g of ethylene oxide, 9.8 g of methyl glycidyl etherand 5 g of allyl glycidyl ether was introduced at room temperature. Thetemperature of the reactor was raised to 120° C. and maintained at thatlevel for 22 hours. A decrease of the pressure from 10.3×10⁵ to 1.4×10⁵Pa was noted. 100 mg of a sulfide of3-tert-butyl-4-hydroxy-5-methyl-phenyl in solution in 50 ml of toluenewere thereafter added. The copolymer was recovered after evaporation ofthe solvent with a yield of 85%, the essential portion of the lossesbeing due to the fact that a small quantity of copolymer remains stuckon the wall of the reactor. A DSC analysis shows a melting peak T_(m) at30° C. and a glass transition temperature Tg=−61° C. The weight averagemolecular weight, determined by steric exclusion chromatography (SEC),is about 80,000, with a polydispersity index of 1.9.

Example 3

In a reactor which is free from traces of water and impurities, 250 mlof toluene, 10⁻³ mole of potassium t-butanolate and 3 ml of THF wereadded. The temperature of the reactor was raised to 110° C. and,thereafter, a mixture of 90.2 g of ethylene oxide, 8.4 g of methylglycidyl ether and 1.9 g of allyl glycidyl ether was added by means of aburette under pressure. The temperature of the reactor was raised to120° C. and was kept at that level for 22 hours, during which a pressuredecrease from 7.7×10⁺⁵ to 2.6×10⁺⁵ Pa was noted. The content of thereactor was thereafter poured into a glass flask containing 100 mg of3-tert-butyl-4-hydroxy-5-methyl-phenyl sulfide under an argonatmosphere. 94.4 g of copolymer were recovered after evaporation of thesolvent, which corresponds to a yield of 94%, not taking into accountthe copolymer which was not extracted from the reactor. A DSC analysisshows a melt peak T_(m) at 33° C. and a glass transition Tg=−59° C. Theweight average molecular weight, determined by steric exclusionchromatography (SEC), was about 110,000, with a polydispersity of 2.2.

Example 4

A material prepared according to example 3 was used to produce a lithiumbattery operating at 60° C. with a composite TiS₂ cathode. The polymerelectrolyte was prepared by dissolving the copolymer, 2% by weight ofbenzoyl peroxide and the salt lithium bis(trifluorosulfonyl)imide (TFSI)in acetonitrile in a ratio 0/Li of 30/1. The solution was thereafterpoured in the form of a film having a thickness of 25 μm and the filmwas dried under vacuum at 90° C. Similarly, the composite electrode wasprepared on nickel by the solvent method utilizing close to 5% ofShawinigan black, some copolymer and TiS₂, so as to give a compositeelectrode whose capacity is 3 Cb per cm². The battery was mounted with alithium anode 22 μm thick by consecutive pressing of the films undervacuum at 85° C., after which it was cycled at 60° C. The utilizationwas maintained at more than 85% during more than 150 cycles carried at adischarge rate of 6 hours (C/6) and at a charge rate (C/12) without anyloss of utilization. This test confirms the electrochemical stability ofthe high molecular weight copolymers of the present invention.

Example 5

Into a reactor 100 ml of THF, 0.1 g of potassium-butanolate, 80 g ofethylene oxide (OE) and 9 g of allyl glycidyl ether were introduced(AGE), which corresponds to a OE/AGE ratio of 22. The reactortemperature was raised to 120° C. and was maintained at that level for 6hours during which a pressure decrease from 7×10⁵ Pa to 1.3×10⁵ Pa wasnoted. At the end of the reaction, the reaction mixture was deactivatedby introducing methanol into the reactor and some3-tert-butyl-4-hydroxy-5-methyl-phenyl sulfide was added. 87 g ofcopolymer were recovered, hereinafter designated by Anio₂₂,corresponding to a nearly quantitative yield. The characteristics of thecopolymer are the following: M_(W)=120,000 g; polydispersity indexI=M_(W)/Mm=1.9; Tg=−62° C.; T_(m)=35.6° C.; rate of crystallinityX=0.37.

The crystallinity rate is measured in the following way: one measures byDSC the heat of fusion of the copolymer, and divides it by the heat offusion of the same amount of pure PEO.

The average number of OE units between two AGE units in the copolymerAnio₂₂ was evaluated in the following manner, presuming that there is aregular distribution of the AGE units in the copolymer chain, namely 22OE units between 2 AGE units. It was assumed that the meltingtemperature of a copolymer OE/AGE in which n OE units are found between2 AGE units is substantially identical to the melting temperature of apolyethylene glycol (PEG) whose average molecular weight Mw issubstantially identical to that of a sequence —(OE)-_(n)-, i.e. Mw=n×44.A determination was then made of the melting temperature of a number ofPEG samples having different molecular weights and a curve T_(m) isproduced as a function of M_(W). FIG. 1 represents the variation of themelting temperature T_(m), expressed in ° C., as a function of theaverage molecular weight, expressed in grams, for a PEG (blacktriangles). It was noted that the melting temperature of the PEGincreases very rapidly with the lengths of the chain to reach 65° C. formolar weights close to 8,000 g. In addition, according to this curve,the PEG whose melting temperature is close to 35° C. has an averagemolar mass of about 1,000 g, which corresponds to about 22 OE units.

Three similar evaluations were carried out with copolymers obtained byutilizing such quantities of monomers that the molar ratios OE/AGE arerespectively 23, 34 and 45 (copolymers respectively designated Anio₂₃,Anio₃₄, and Anio₄₅). The melting temperature of each copolymer wasdetermined and inserted on the curve of FIG. 1 (white squares). It wasconfirmed that the melting temperature of a copolymer Anion issubstantially identical to that of PEG having a molecular weightsubstantially equal to n×44. These tests thus confirm the regulardistribution of the monomeric units in the copolymers OE/AGE obtained bythe polymerization process according to the present invention.

Example 6

The copolymer Anio₂₂ of example 4 was cross-linked at 70° C. in thepresence of 2 weight % of benzoyl peroxide with respect to thecopolymer. The cross-linked network obtained has a melting temperatureT_(m)=28° C., and a crystallinity rate=0.20.

Many electrolytes were prepared in the form of a film starting from thecopolymer Anio₂₂. Solutions of copolymer Anio₂₂, lithiumtrifluorosulfonyl imide (TFSI) and benzoyl peroxide in acetonitrile, inwhich the TFSI content varies from one solution to the other, wereprepared, and each solution was poured onto a support. After evaporationof the solvent, each film of copolymer obtained was cross-linked byheating, it was kept under vacuum during several days, and was preservedin a glove box. The conductivity was determined at differenttemperatures between 20° C. and 84° C.

FIG. 2 represents the variation of conductivity σ, expressed in Ω⁻¹cm⁻¹,as a function of the temperature, expressed in ° C., for differentelectrolytes. The clear circles correspond to an electrolyte consistingof the copolymer Anio₂₂ and the salt TFSI in which the ratio O/Li=14;the black circles correspond to an electrolyte consisting of copolymerAnio₂₂ and TFSI in which the ratio O/Li=16.7; the clear trianglescorrespond to an electrolyte consisting of a poly(oxyethylene) whosemolecular weight is 5×10⁶ g/mole and in which the TFSI salt content issuch that O/Li=8, usually considered as reference polymer having thebest ionic conductivity.

The conductivities obtained for the electrolytes of the invention are,within the entire temperature range explored, higher than those notedfor the reference complex poly(oxyethylene).

A DSC analysis has confirmed the completely amorphous character of theelectrolytes.

The Table which follows shows glass transition temperature Tg (° C.) fordifferent salt concentrations, indicated by the atomic ratio O/Li, andshows that the glass transition temperature of an electrolyte preparedfrom a copolymer according to the invention slowly decreases as the saltconcentration increases. O/Li Tg (° C.) 8 −33.6 9 −34.6 13 −38.6 16.7−46 22 −49.2 27.5 −51.2

The range of electrochemical stability of the electrolytes networkAnio₂₂/LiTFSI was measured by cyclic voltametry on a platinummicro-electrode at 81° C. The volt-amperograms obtained show the lithiumdeposit, in the form of a sharp peak at about OV vs lithium. A returnsweep shows the reoxidation of the alloys and the intermetalliccompounds, formed between platinum and lithium. No peak resulting fromthe oxidation or the reduction of the copolymer was noted in the powerrange explored, namely from 0 to +3.9 V vs Li/Li+.

A thermogravimetric analysis shows that the electrolytes are stable upto 240° C., which is amply sufficient when they are used in all solidlithium batteries, since melting of lithium takes place already at 180°C.

Example 7

150 ml of THF, 0.12 g of potassium t-butanolate, 89 g of ethylene oxide(OE), 9.8 g of epoxyhexene and 8.4 g of an epoxide CH₂—CHR′—O in whichR′ represents a group CH₂—O—CF₂—CF₂—SO₃—K were added to a reactor. Thetemperature of the reactor was raised to 110° C. and maintained at thatlevel for 12 hours. Then, the reaction mixture was deactivated byintroducing methanol into the reactor and a precipitate was formed inhexane, which gave 103 g of a polymer having ionophoric functions. Theyield is near 96%.

An analysis by steric exclusion chromatography (SEC), on two columns ofnominal steric exclusion 10 nm size of a solution obtained by dissolvingthe precipitate in THF showed no residues of the ionic monomerCH₂—CHR′—O which would not have reacted.

A cross-linked membrane was produced by adding 1% by weight (withrespect to the polymer) of benzoyl peroxide to a solution of thecopolymer in a solvent and by heating at 80° C. during two hours. Theconductivity of the membrane obtained is 10⁻⁵ S.cm⁻¹ at 37° C. and 10⁻⁴S.cm⁻¹ at 77° C.

A DSC analysis shows a melting point of 24° C. and a glass transitiontemperature of −55° C.

A sample of the cross-linked membrane was swelled three times with asolution of (CF₃SO₂)₂NLi in acetonitrile, in order to exchange thecations K⁺ with cations Li⁺. The membrane was thereafter put on afiltrating crucible and the solution of acetonitrile was removed byfiltration. The membrane was then washed three times in 50 ml ofacetonitrile in order to remove any trace of free salt, and this wasfollowed by careful drying. Its conductivity was then 10⁻⁵ S.cm⁻¹ at 45°C. and 10⁻⁴ S.cm⁻¹ at 95° C. Its melting temperature was 25° C. and itsglass transition temperature was −57° C.

Example 8

An ionically conductive unipolar electrolyte was prepared byco-cross-linking the copolymer Anio₂₂ prepared according to example 4with an ionophoric compound carrying a double bond of the allyl type,CH₂═CH—CH₂—O—CF₂—CF₂—SO₃Li. Cross-linking was carried out with 2% byweight of benzoyl peroxide at 70° C. for 3 hours. The quantity ofionophoric compound added corresponds to a ratio O/Li of 15. An analysisof the solvents after washing the cross-linked membrane shows that about90% of the salt was incorporated into the network. In the network, theratio O/Li is therefore 17.

A DSC analysis shows a melting peak at 19° C. and a glass transitiontemperature of −58° C.

The electrolyte reaches a conductivity of 10⁻⁵ S.cm⁻¹ at 33° C. and 10⁻⁴S.cm⁻¹ at 70° C.

The membrane was thereafter swelled by incorporating 30% by weight of amixture in a 2/1 molar proportion of propylene carbonate and ethylenecarbonate. The conductivity then reached 5.10⁻⁵ S.cm⁻¹ at 20° C. and10⁻³ S.cm⁻¹ at 60° C.

1. A statistical copolymer comprising ethylene oxide, —O—CH₂—CHR— units,in which R is a substituent having a reactive function, which iscross-linkable by free radical process, R may vary from one unit to theother and —O—CH₂—CHR′— units in which R′ is a substituent containing noreactive function which is cross-linkable by free radical process, R′may vary from one unit to the other, said statistical copolymer has apolydispersity index I=Mw/Mn lower than or equal to 2.2, a randomdistribution of monomer units and an average molecular mass in Mn numberat least equal to 20,000.
 2. (canceled)
 3. The statistical copolymeraccording to claim 1, wherein the average molecular mass in Mn number ishigher than or equal to 100,000.
 4. The statistical copolymer accordingto claim 1, wherein the substituent R is a radical having the formulaCH₂═CH—(CH₂)_(q)—(O—CH₂)_(p) in which 1≦q≦6 and p=0 or 1, or formulaCH₃—(CH₂)_(y)—CH═CH—(CH₂)_(x)—(OCH₂)_(p), in which 0≦x+y≦5 and p=0 or 1.5. The statistical copolymer according to claim 1, wherein thesubstituent R′ is selected among alkyl radicals; among alkoxy radicals;among alkyl(perfluoroalkyl sulfonate) ether radicals; or among radicalsincorporating an ionophoric function in which the negative charge iscarried by the bis(trifluoromethylsulfonyl) methylide carbanion—C(SO₂CF₃)₂M″, wherein M″ represents a metallic cation.
 6. Thestatistical copolymer according to claim 5, where the substituent R′ isselected among alkyl radicals having 1 to 16 carbon atoms, and a radical—(CH₂)_(n)—O—((CH₂)_(m)—O)_(p)—CH₃ radicals, in which 0≦n≦4; 1≦m≦4 and0≦p≦20.
 7. The statistical copolymer according to claim 5, wherein M″represents a monovalent cation or a cation of an alkali metal.
 8. Thestatistical copolymer according to claim 1, characterized in that itcontains at least 70 mole % of ethylene oxide units, about 2 to about 30mole % of —O—CH₂—CHR′— units and about 0.05 to about 10 mole % of—O—CH₂—CHR— units. 9-26. (canceled)
 27. The statistical copolymeraccording to claim 1 comprising deactivated terminal functions.